Among document users, a user may wish to retrieve documents satisfying a retrieval condition as a document group from a plurality of documents that are divided into a plurality of document groups that include mutually related documents. For example, the user may, when performing a retrieval based on the retrieval condition “A AND B”, wish to retrieve not only a document group including documents that satisfy both of the conditions A and B, but also a document group including documents which satisfy either one of the conditions A and B. Document groups may be registered including mutually related documents or that performs a retrieval in a manner that takes a document group including mutually related documents as a target. A user may wishes to retrieve documents that satisfy a retrieval condition as a document group from a plurality of documents divided into a plurality of document groups that include mutually related documents.